Kohana框架as_array转换json数据

$result = Db::select('id', 'username')
                ->from('kohana')
                ->execute();
$r = $result->as_array();
var_dump($r);
$str = json_decode($r);
echo $r

这里用到db::操作数据库，很方便。orm好象还不能这样转换json

ORM as_array()

ORM::factory('user')->find_all ()->as_array ('username','username');